Process for preparing and purifying fatty acids

ABSTRACT

There is provided a process for purifying a fatty acid, which process comprises reacting a fatty acid with a lithium salt in a first solution and under conditions to allow formation of a precipitate of a lithium salt of the fatty acid; isolating the precipitate; dissolving the precipitate in a second solution followed by separation of the organic and aqueous layers so formed; and evaporating the organic layer to isolate the purified fatty acid. There is also provided a process for increasing the length of a fatty acid, and the use of a lithium salt to purify a fatty acid.

BACKGROUND TO THE INVENTION

The invention is concerned with the preparation and purification offatty acids. Fatty acids are aliphatic monocarboxylic acids which arecommonly derived from animal and vegetable sources. As well as being asource of energy, fatty acids play many other key roles in the body.They can help to regulate healthy lipid levels, and are involved ininflammatory responses. They are also important in the blood, regulatingclotting and blood pressure.

A number of fatty acids can be synthesised by the body in vivo. Howeversome, designated “essential fatty acids”, cannot. Essential fatty acidsinclude the short chain polyunsaturated fatty acids (SC-PUFAs) linoleicacid and α-linolenic acid, as well as long chain polyunsaturated fattyacids (LC-PUFAs) which can be prepared from these SC-PUFAs. These twocategories are generally split into two further categories, the ω-3 (or“Omega 3”) fatty acids, and the ω-6 (or “Omega 6”) fatty acids.Representative LC-PUFAs include the ω-3 fatty acids eicosapentaenoicacid (EPA) and docosahexaenoic acid (DHA), and the ω-6 fatty acidsgamma-linolenic acid (GLA), dihomo-gamma-linolenic acid (DGLA) andarachidonic acid (AA).

As essential fatty acids cannot be synthesised by the body in vivo theymust be provided by diet. There is therefore a need to develop andimprove processes and techniques for isolating and purifying theseessential fatty acids. There is also a need to develop and improveprocesses for converting essential fatty acids into other fatty acids,in order to provide a wide range of compounds and supplements necessaryto meet the requirements of individuals. Further, a number of PUFAproducts are needed in purified form as they are pharmaceuticallyactive.

Novel processes for purifying fatty acids have now been found which canlead to simplified production and/or increased purity and/or easierscale-up of process. The novel purification processes can be used inisolation on a prepared fatty acid, or can be incorporated into a longerprocess for the preparation of said fatty acid. These processes help toremove non-acidic impurities from the fatty acids.

SUMMARY OF THE INVENTION

The invention provides the use of a lithium salt to purify a fatty acid.

The invention further provides a process for purifying a fatty acid,which process comprises:

-   -   (a) reacting a fatty acid with a lithium salt in a first        solution and under conditions to allow formation of a        precipitate of a lithium salt of the fatty acid;    -   (b) isolating the precipitate;    -   (c) dissolving the precipitate in a second solution which is        capable of generating two immiscible layers upon dissolution of        the precipitate, the two immiscible layers being an organic        layer and an aqueous acidic layer;    -   (d) separating the two immiscible layers formed upon dissolution        of the precipitate; and    -   (e) evaporating the organic layer to isolate the purified fatty        acid.

There is also provided a process for preparing a fatty acid, whichprocess comprises:

-   -   (a) decarboxylating a malonic acid derivative of formula        R—CH₂CH(CO₂H)₂, wherein R is a fatty acid residue, to form a        fatty acid of formula RCH₂CH₂CO₂H;    -   (b) subjecting the fatty acid thus prepared to a process for        purifying a fatty acid as described above.

There is further provided a process for extending the length of a fattyacid, which process comprises:

-   -   (a) reducing a fatty acid of formula R—CO₂H or a fatty acid        ester of formula R—CO₂R¹, wherein R is a fatty acid residue and        R¹ is a C₁₋₆ alkyl group, to an alcohol of formula R—CH₂OH;    -   (b) sulfonating the alcohol to form a sulfonate of formula        R—CH₂OSO₂R², wherein R² is a C₁₋₆ alkyl or C₆₋₁₀ aryl group;    -   (c) reacting the sulfonate with a malonate ester derivative and        hydrolysing the resulting product to form a malonic acid        derivative of formula R—CH₂CH(CO₂H)₂;    -   (d) decarboxylating the malonic acid derivative to form a fatty        acid of formula R—CH₂CH₂CO₂H; and    -   (e) subjecting the fatty acid thus prepared to a process for        purifying a fatty acid as described above.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, a C₁₋₆ alkyl group is a linear or branched alkyl groupcontaining from 1 to 6 carbon atoms, for example a C₁₋₄ alkyl groupcontaining from 1 to 4 carbon atoms. Examples of C₁₄ alkyl groupsinclude methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl and t-butyl.For the avoidance of doubt, where two alkyl moieties are present theymay be the same or different.

As used herein, a C₂-C₄ alkenyl group is a linear or branched alkenylgroup having at least one double bond of either cis or transconfiguration where applicable and containing from 2 to 4 carbon atoms,for example —CH═CH₂ or —CH₂—CH═CH₂; —CH₂—CH₂—CH═CH₂, —CH₂—CH═CH—CH₃,—CH═C(CH₃)—CH₃ and —CH₂—C(CH₃)═CH₂, preferably a C₂ alkenyl group having2 carbon atoms. For the avoidance of doubt, where two alkenyl groups arepresent in a compound of the present invention, they may be the same ordifferent.

As used herein, a halogen atom is typically chlorine, fluorine, bromineor iodine.

As used herein, a C₁-C₄ alkoxy group or C₂-C₄ alkenyloxy group istypically a said C₁-C₄ alkyl group or a said C₂-C₄ alkenyl grouprespectively which is attached to an oxygen atom.

A haloalkyl, haloalkenyl, haloalkoxy or haloalkenyloxy group istypically a said alkyl, alkenyl, alkoxy or alkenyloxy group respectivelywhich is substituted by one or more said halogen atoms. Typically, it issubstituted by 1, 2 or 3 said halogen atoms. Preferred haloalkyl andhaloalkoxy groups include perhaloalkyl and perhaloalkoxy groups, such as—CX₃ and —OCX₃ wherein X is a said halogen atom, for example chlorineand fluorine.

As used herein, a C₁-C₄ alkylthio or C₂-C₄ alkenylthio group istypically a said C₁-C₄ alkyl group or a C₂-C₄ alkenyl group respectivelywhich is attached to a sulfur atom, for example —S—CH₃.

As used herein, a C₁-C₄ hydroxyalkyl group is a C₁-C₄ alkyl groupsubstituted by one or more hydroxy groups. Typically, it is substitutedby one, two or three hydroxy groups. Preferably, it is substituted by asingle hydroxy group.

As used herein, a C₆₋₁₀ aryl group is a phenyl group or a naphthylgroup. For the avoidance of doubt, where two aryl groups are presentthey may be the same or different.

Unless otherwise specified, C₆₋₁₀ aryl groups can be unsubstituted orsubstituted with 1, 2, 3 or 4 substituents which are the same ordifferent and are chosen from halogen atoms and C₁₋₄ alkyl, C₂₋₄alkenyl, C₁₋₄ alkoxy, C₂₋₄ alkenyloxy, C₁₋₄ haloalkyl, C₂₋₄ haloalkenyl,C₁₋₄ haloalkoxy, C₂₋₄ haloalkenyloxy, hydroxyl, mercapto, cyano, nitro,C₁₋₄ hydroxyalkyl, C₂₋₄ hydroxyalkenyl, C₁₋₄ alkylthio, C₂₋₄ alkenylthioand —NR″R″ groups wherein each R′ and R″ is the same or different andrepresents hydrogen or C₁₋₄ alkyl. Where a substituent on an aryl groupis selected from phenyl, carbocyclyl, heterocyclyl, heteroaryl,—COR^(A), —SO₂R^(A), —CONH₂, —SO₂NH₂, —CONHR^(A), —SO₂NHR^(A),—CONR^(A)R^(B) and —SO₂NR^(A)R^(B), preferably only one such substituentis present. Preferably the C₆₋₁₀ aryl groups are unsubstituted orsubstituted with 1 or 2, preferably 1, unsubstituted substituent.Preferred substituents include C₁₋₄ alkyl, C₁₋₄ alkoxy, C₁₋₄ alkylthioand hydroxyl groups. More preferred substituents include halogen atoms,and C₁₋₄ alkyl, C₁₋₂ alkoxy and hydroxy groups. Most preferably the arylgroups are unsubstituted.

As used herein, a C₃₋₇ carbocyclic group is a non-aromatic saturated orunsaturated hydrocarbon ring having from 3 to 7 carbon atoms. Preferablyit is a saturated or mono-unsaturated hydrocarbon ring (i.e. acycloalkyl moiety or a cycloalkenyl moiety) having from 3 to 7 carbonatoms, more preferably having from 3 to 6 carbon atoms. Examples includecyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl and theirmono-unsaturated variants, more particularly cyclopentyl and cyclohexyl.A C₃₋₇ carbocyclyl group or moiety also includes C₃₋₇ carbocyclyl groupsor moieties described above but wherein one or more ring carbon atomsare replaced by a group —C(O)—. Preferably the carbocyclic groups do nothave any ring carbon atoms replaced by a group —C(O)—.

As used herein, a 5- or 6- membered heterocyclyl group is anon-aromatic, saturated or unsaturated C₅₋₆ carbocyclic ring in whichone or more, for example 1, 2, 3 or 4, of the carbon atoms are replacedwith a moiety selected from N, O, S, S(O) and S(O)₂, and wherein one ormore of the remaining carbon atoms is optionally replaced by a group—C(O)— or —C(S)—. When one or more of the remaining carbon atoms isreplaced by a group —C(O)— or —C(S)—, preferably only one or two (morepreferably two) such carbon atoms are replaced. Suitable heterocyclylgroups include pyrrolidinyl, pyrrolinyl, pyrrolyl, tetrahydrofuranyl,dihydrofuranyl, furanyl, tetrahydrothiophenyl, dihydrothiophenyl,thiophenyl, imidazolidinyl, pyrazolidinyl, imidazolyl, imidazolinyl,pyrazolyl, pyrazolinyl, oxazolidinyl, isoxazolidinyl, oxazolyl,oxazolinyl, isoxazolyl, isoxazolinyl, thiazolidinyl, isothiazolidinyl,thiazolyl, thiazolinyl, isothiazolyl, isothiazolinyl, dioxolanyl,oxathiolanyl, dithiolanyl and thiophenyl.

As used herein the term “salt” includes base addition, acid addition andquaternary salts. Exemplary salts include those formed with bases suchas alkali metal hydroxides, e.g. lithium, sodium and potassiumhydroxides; alkaline earth metal hydroxides e.g. calcium, barium andmagnesium hydroxides; with organic bases e.g. N-methyl-D-glucamine,choline tris(hydroxymethyl)amino-methane, L-arginine, L lysine, N-ethylpiperidine, dibenzylamine and the like.

Unless otherwise defined, as used herein the term “fatty acid”represents a C₄₋₂₆ aliphatic monocarboxylic acid, more preferably aC₁₀₋₂₄ aliphatic monocarboxylic acid, more preferably a C₁₄₋₂₄ aliphaticmonocarboxylic acid. The fatty acids which are the products of theprocesses mentioned herein preferably contain from 16 to 24 carbonatoms. Fatty acids are derived from or contained in many sources. Forexample they can be derived from or contained in esterified form in, ananimal or vegetable fat, oil, or wax. They can also be prepared fromother, shorter chain fatty acids in accordance with one of the processesof the invention.

The fatty acids used in the invention have hydrocarbon chains which arestraight or branched. Preferably the hydrocarbon chains are straight.The hydrocarbon chains can also contain, within the chain, a C₃₋₇carbocyclyl or 3- to 7-membered heterocyclyl ring. Preferably, however,the hydrocarbon chains do not contain carbocyclic or heterocyclyl rings.The hydrocarbon chains can also contain, within their backbone, one ormore, preferably one, oxygen atom. Preferably, however, the hydrocarbonchains do not contain oxygen atoms.

As used herein, the term “fatty acid ester” represents an ester of afatty acid described above. Preferably the fatty acid ester is an alkylester with the alkyl group being a C₁₋₆ alkyl group. Preferred fattyacid esters thus include esters of formula R—CO₂R¹ wherein R is a fattyacid residue and R¹ is a C₁₋₆ alkyl group. Preferably R¹ isunsubstituted. Preferably R¹ is a C₁₋₄ alkyl group, more preferably amethyl or ethyl group, most preferably an ethyl group.

As used herein, the term “fatty acid residue” refers to the hydrocarbyltail of a fatty acid as described above. Specifically the fatty acidresidue corresponds to a fatty acid excluding the terminal carboxylicacid group. Accordingly, a fatty acid of formula R—CO₂H contains thefatty acid residue R. In the following discussion the fatty acid residuewill be described with reference to the fatty acid from which it isderived. For the avoidance of doubt, the fatty acid residue can also bederived from a fatty acid ester of formula R—CO₂R′ where R′ is the alkylgroup of the ester.

Preferably R is a fatty acid residue formed from a fatty acid containing14 to 22 carbon atoms, more preferably from 16 to 22 carbon atoms, mostpreferably from 18 to 20 carbon atoms.

Preferably R is a fatty acid residue formed from a fatty acid which isfully saturated or contains from 1 to 6 centres of unconjugatedunsaturation. The centres of unconjugated unsaturation representolefinic (—CH═CH—) and/or acetylenic (—C≡C—) groups which are arrangedsuch that their delocalised electrons are not in conjugation withanother centre of unsaturation. Preferably the fatty acid contains 2, 3,4, 5 or 6 more preferably 2, 3, 4 or 5, more preferably 2, 3 or 4, mostpreferably 3 or 4 centres of unconjugated unsaturation. It is preferredthat the centres of unconjugated unsaturation are olefinic groups. Aswill be appreciated, for a hydrocarbyl chain of a given length, theremay be multiple arrangements of the unconjugated unsaturation along thechain. For example, fatty acids designated as ω-3 (or “Omega 3”) fattyacids contain a final carbon-carbon double bond in the n−3 position,i.e. the third bond from the methyl end of the fatty acid is acarbon-carbon double bond. Fatty acids designated as ω-6 (or “Omega 6”)fatty acids contain a final carbon-carbon double bond in the n−6position, i.e. the sixth bond from the methyl end of the fatty acid is acarbon-carbon double bond. Preferably R is a fatty acid residue formedfrom an ω-3 or ω-6 fatty acid, also referred to as ω-3 fatty acidresidues or w-6 fatty acid residues.

Most preferred R groups are those derived from ω-3 or ω-6 fatty acidshaving from 16 to 22 carbon atoms, more preferably from 18 to 20 carbonatoms, and containing from 2 to 5, more preferably 2 to 4, mostpreferably 3 or 4, unconjugated olefinic groups. Exemplary fatty acidresidues include the C₁₇H₂₉-residue from gamma-linolenic acid, theC₁₉H₂₉-residue from eicosapentaenoic acid, the C₁₇H₂₇-residue fromstearidonic acid, the C₁₇H₃₁-residue from linoleic acid, and theC₁₇H₂₉-residue from alpha-linolenic acid. Preferred fatty acid residuesare the C₁₇H₂₉-residue from gamma-linolenic acid, the C₁₉H₂₉-residuefrom eicosapentaenoic acid, and the C₁₇H₂₇-residue from stearidonicacid. Most preferred fatty acid residues are the C₁₇H₂₉-residue fromgamma-linolenic acid, and the C₁₉H₂₉-residue from eicosapentaenoic acid.For the avoidance of doubt, the structures of gamma-linolenic acid,eicosapentaenoic acid and stearidonic acid are as follows:

The fatty acids which are purified in accordance with the purificationprocess of the invention are preferably C₁₆₋₂₄ fatty acids. They arepreferably prepared via a chain extension process (e.g. via a malonatechain extension process) from the fatty acids of formula R—CO₂H where Ris a fatty acid residue described above. Accordingly, the fatty acidswhich are purified in accordance with the invention are preferably offormula R—CH₂—CH₂—CO₂H, generated by a chain extension process of afatty acid of formula R—CO₂H. The group R in the fatty acids of formulaR—CH₂—CH₂—CO₂H is preferably as described earlier.

These fatty acids which are purified in accordance with the purificationprocess of the invention are preferably ω-3 or ω-6 fatty acids havingfrom 16 to 24 carbon atoms, more preferably from 20 to 22 carbon atoms,and containing 2, 3, 4, 5 or 6, more preferably 2, 3, 4 or 5, morepreferably 2, 3 or 4, most preferably 3 or 4, unconjugated olefinicgroups. Exemplary fatty acids include the ω-6 fatty acidsdihomo-gamma-linolenic acid (DGLA) and eicosadienoic acid, and the ω-3fatty acids docosapentaenoic acid (DPA, sometimes referred to as DEPA),eicosatetraenoic acid (ETA) and eicosatrienoic acid. A preferred ω-6fatty acid is dihomo-gamma-linolenic acid (DGLA). Preferred ω-3 fattyacids are docosapentaenoic acid (DPA) and eicosatetraenoic acid (ETA),more preferably docosapentaenoic acid (DPA). For the avoidance of doubt,the structures of DGLA (ω-6), DPA (ω-3) and ETA (ω-3) are as follows,with carbon atom numbering being included for information purposes:

As described earlier, the invention provides the use of a lithium saltto purify a fatty acid.

Typically, the lithium salt is other than lithium aluminium hydride.Preferably, the lithium salt is not hydride-yielding.

Typically, the fatty acid is a single fatty acid as defined above, i.e.the fatty acid is not a mixture of fatty acids. Thus, typically, thelithium salt is added to a mixture of the single fatty acid andnon-acidic impurities. These non-acidic impurities are typicallyproduced during the process of synthesising the fatty acid, for exampleas described herein. Thus, the present invention typically does notinvolve separating fatty acids from one another.

Typically, the lithium salt is lithium bicarbonate, lithium carbonate orlithium hydroxide. Preferably, the lithium salt is lithium hydroxide,more preferably lithium hydroxide hydrate.

Typically, the fatty acid has 1 to 6 centres of unconjugatedunsaturation as defined herein. Preferably the fatty acid is an ω-3 orω-6 fatty acid.

Preferably the purification process occurs after the last of foursynthetic stages:

Stage 1:

The lithium salt and crude fatty acid are combined in a first solution,and are held under conditions which allow formation of a precipitate ofa lithium salt of the fatty acid. Suitable lithium salts include lithiumbicarbonate, lithium carbonate and lithium hydroxide. Preferably thelithium salt is lithium hydroxide. The lithium salt may be supplied inthe form of a hydrate, for example lithium hydroxide hydrate. It may beadded to the first solution in its hydrate form, but is preferably firstdissolved in a suitable solvent such as water. Preferably the firstsolution contains a ketone, more preferably acetone. The temperature ofthe reaction is preferably between about −30 and about 30° C.

Stage 2:

The precipitated lithium salt is then isolated. Any suitable method forisolating a solid precipitate from solution can be used, for examplefiltering. The precipitate is optionally washed with further solvent(e.g. the same solvent as in Stage 1, preferably acetone), and anysolvent evaporated.

Stage 3:

The isolated precipitate is then dissolved in a second solution. Thesecond solution is chosen such that, upon dissolution of theprecipitate, two immiscible layers are formed. The two immiscible layersare a polar layer which is an aqueous acidic layer, and a non-polar,organic layer. The aqueous acidic layer is preferably an aqueoussolution of a strong mineral acid such as hydrochloric acid. Thenon-polar, organic layer is suitably an ether A preferred ether for thenon-polar layer is t-butyl methyl ether.

Stage 4:

In this stage the two immiscible layers formed upon dissolution of theprecipitate are separated. Conventional separating techniques can beused, for example using a simple separating funnel. Following separationof the two immiscible layers, the resulting organic layer may optionallybe washed with water and dried (e.g. using Na₂SO₄ followed byfiltration).

Stage 5:

The solvent is removed from the organic layer by evaporation to isolatethe purified fatty acid. The purified fatty acid has a higher level ofpurity compared to the crude product. For example, the purity can beincreased by about 1% or greater, preferably about 2% or greater, morepreferably about 5% or greater. The product obtained from this stage maybe colourless or coloured. For example, a pale yellow oil can beobtained. The product is optionally further decolourised, suitablyemploying chromatographic silica in an appropriate solvent. For example,decolourisation can be achieved by stirring with 10-20% by weightchromatographic silica in hexane.

The crude fatty acid used in the above purification process can bederived from a number of sources. One suitable method for preparing thefatty acid is via decarboxylation of a malonic acid derivative. Forexample, a malonic acid derivative of formula RCH₂CH(CO₂H)₂ can bedecarboxylated to form a fatty acid of formula RCH₂CH₂CO₂H. Theresulting fatty acid of formula RCH₂CH₂CO₂H can then be subjected to thepurification process described above.

The malonic acid derivative described above can be derived from a numberof sources. For example, it can be provided in crystalline form, havingbeen isolated and optionally purified from an earlier process. However,it is preferably provided as a crude reaction product without havingundergone purification. For example, it can be provided from reaction ofa sulfonate to a malonic ester derivative, suitably via a malonic esterintermediate and subsequent hydrolysis.

The invention also provides a process for extending the length of afatty acid. In particular, the process can be used to extend a fattyacid by two carbon atoms. The two carbon atoms are effectively insertedbetween the fatty acid residue R and the carboxylic acid group. Theextension process comprises four separate stages:

Stage I

The starting fatty acid is of formula R—CO₂H. Alternatively, thecorresponding fatty acid ester can be used, having formula R—CO₂R¹wherein R is the fatty acid residue and R¹ is a C₁ ₋₆ alkyl group.Preferred R¹ groups are C₁ ₋₄ alkyl groups, more preferably methyl orethyl, most preferably ethyl.

The fatty acid or fatty acid ester is reduced to form the correspondingfatty alcohol of formula R—CH₂OH. Suitable reduction techniques are wellknown, and skilled person will readily be able to choose appropriatereducing agents and reaction conditions. Reducing agents include Red-Al(sodium bis(2-methoxyethoxy)aluminumhydride), DIBAL (Diisobutylaluminiumhydride) and lithium aluminium hydride. The reducing agents are used inconjunction with an appropriate solvent, with suitable inert solventsincluding ethers and aromatic hydrocarbons and derivatives thereof.Preferred solvents include diethyl ether, tetrahydrofuran and toluene.The temperature of the reaction can vary, with a suitable temperaturerange being from 0 to 35° C. When the starting material is a fatty acid,then this reduction reaction evolves hydrogen. The hydrogen must becarefully and safely removed. Use of a fatty acid ester startingmaterial reduces the amount of hydrogen liberated, as the only hydrogenproduced results from decomposition of excess reducing agent.

The preferred reducing agent is lithium aluminium hydride. This can beadded to the reaction in various forms, for example as a solid or insolution. Addition in solid form may be appropriate for small-scaleproduction. For scaled-up processes it is preferred to employ lithiumaluminium hydride in solution, leading to improved and safer handling.

Stage II

The alcohol prepared in Stage I is subsequently sulfonated to form afatty acid sulfonate of formula R—CH₂OSO₂R², wherein R² is a C₁₋₆ alkylor C₆₋₁₀ aryl group. Preferably R² is a C₁₋₆ alkyl, more preferably aC₁₋₄ alkyl, most preferably methyl. When R² is a C₆₋₁₀ aryl group, thearyl group is preferably phenyl. The aryl groups are unsubstituted orsubstituted, as described earlier. A most preferred substituent ismethyl. Suitable sulfonating agents are chosen appropriately, forexample methanesulfonyl chloride, phenylsulfonyl chloride and4-methylphenylsulfonyl chlorides are preferred sulfonating agents, withmethanesulfonyl chloride being particularly preferred. Preferably thereaction occurs in the presence of a tertiary base such as pyridine,2,4,6-trimethylpyridine or triethylamine. The temperature of thereaction is preferably between about 0 and 40° C.

The reaction optionally occurs in a suitable solvent. Chlorinatedsolvents (e.g.

dichloromethane) are conventionally used in this type of sulfonationreaction. However, minimisation or avoidance of the use of chlorinatedsolvent is preferred. One way to reduce or avoid use of a chlorinatedsolvent is to use pyridine as a base.

Stage III

The fatty acid sulfonate prepared in Stage II is subsequently reactedwith a malonate ester derivative, the product thereof being hydrolysedto form a malonic acid derivative of formula R—CH₂CH(CO₂H)₂. The initialreaction preferably takes place in an anhydrous alcohol, for exampleabsolute ethanol. The temperature of the reaction is preferably fromabout 60-90° C. The hydrolysis can take place under any suitablehydrolysis conditions, e.g. in aqueous alcohol in the presence of agroup I metal hydroxide. The temperature of the reaction is preferablyfrom about 15-50° C.

Suitable malonate ester derivatives are group I metalo-malonates,including sodio dialkyl malonates, NaCH(CO₂R³)₂ where R³ a C₁₋₆ alkylgroup. Preferably R³ is a C₁₋₄ alkyl group, more preferably ethyl. Thereaction initially produces an ester of formula R—CH₂CH(CO₂R³)₂ which isthen hydrolysed to prepare the malonic acid derivative of formulaR—CH₂CH(CO₂H)₂. Hydrolysis reagents and conditions are well known. Forexample, reaction with a suitable hydroxide (e.g. sodium or potassiumhydroxide) can yield the malonic acid derivative.

The malonic acid derivative of formula R—CH₂CH(CO₂H)₂ can, depending onthe nature of R, be isolated and crystallised. However, some malonicacid derivatives do not crystallise easily or at all. For example, themalonic acid derivative formed from EPA or an ester thereof (i.e. whereR is H₃C—(CH₂—CH═CH)₅-(CH₂)₃-) does not readily crystallise. In thesecircumstances, where a purification process cannot easily be performedon a non-crystallising product, the lithium salt purification processdescribed above provides a convenient method for improving the purity ofthe final fatty acid. The lithium salt purification process avoids theneed to crystallise and purify the malonic acid derivative of formulaR—CH₂CH(CO₂H)₂, and instead allows purification of the final fatty acidproduct instead. Accordingly, the process for extending the length of afatty acid described earlier preferably takes the crude product fromStage III and uses this as the feed for Stage IV, without firstpurifying the product of Stage III.

Stage IV

The malonic acid derivative prepared in Stage III is subsequentlydecarboxylated to form a fatty acid of formula R—CH₂CH₂CO₂H. Standarddecarboxylation techniques can be used, with the evolved carbon dioxidebeing removed (e.g. by vacuum) during the course of the reaction. Forexample, simple heating can achieve decarboxylation. Suitable heatingtemperatures will vary, but general ranges include from 120-180° C., forexample from 130-170° C., preferably from 140-160° C. The temperatureshould be held until the reaction has gone to completion, which will beapparent by the emission of carbon dioxide reducing and eventuallyceasing. A vacuum can also be employed, for example a vacuum of lessthan about 30 mb is suitable.

Stage V

The fatty acid prepared in Stage IV is subsequently purified using theprocess for purifying a fatty acid as described above. Purification bythis method removes non-acidic impurities from the fatty acid. The finalfatty acid product has a higher level of purity compared to the fattyacid formed in Stage IV. The purity of the final fatty acid may have apurity similar to the purity of the initial fatty acid used in Stage 1,i.e. the fatty acid of formula R—CO₂H. If the initial fatty acidcontained non-acidic impurities, then the purity of the final fatty acidwill be greater.

EXAMPLES Example 1 Preparation and purification of DGLA(Icosa-8(Z),11(Z),14(Z)-trienoic acid Example 1a Preparation ofGLAlcohol (Octadeca-6(Z),9(Z),12(Z)-trienol)

To dry fresh tetrahydrofuran (12000 parts, vol) under nitrogen is addedlithium aluminium hydride in tetrahydrofuran (2.4 Molar, 1620 parts,vol). The mixture is cooled to 0-5° C. and GLA (gamma linolenic acid,95-98%, 1112 parts, wt) in dry tetrahydrofuran (2000 parts, vol) isadded over 30-40 min, keeping the temperature at around 3-7° C., withstirring and under a nitrogen stream. The mixture is then stirred at8-12° C. for 1 hr and 12-18° C. for 2 hr under nitrogen. After coolingto 3-5° C., a solution of water (152 parts, vol) in tetrahydrofuran (500parts, vol) is added under a good stream of nitrogen over 15-20 min. Anaqueous solution of sodium hydroxide (2 M, 456 parts, vol) is then addedover 10-15 min. The mixture is stirred at 10-15° C. sealed undernitrogen overnight and then anhydrous sodium sulfate (500 parts, wt) isadded and the mixture stirred for a further 30 min. After filtration,the inorganic solids are washed with tetrahydrofuran (2000 parts, vol).The resulting THF solution is evaporated under vacuum. Any water in theproduct is removed by evaporating with 2×2000 parts, vol. of toluene.There is obtained GLAlcohol (1029 parts, wt, 97.4%) as a pale yellowoil.

Example 1b Preparation of GLAlcohol Methane Sulfonate(Octadeca-6(Z),9(Z),12(Z)-trienyl methane sulfonate

To a stirred mixture of GLAlcohol (1000 parts, wt) and methanesulfonylchloride (456 parts, wt) under nitrogen and at 8-12° C. is added drypyridine (307 parts, wt) over a period of 30-40 min keeping thetemperature below 15° C. The mixture is stirred at this temperature for3-5 hrs and then allowed to warm up to room temperature and stirred overa period of 24-48 hrs. A precipitate of pyridine hydrochloride occurs inthe mixture. The reaction mixture is then diluted with hexane (4000parts, vol), anhydrous sodium sulfate (200 parts, wt) added and theresulting mixture stirred for 1 hr. The precipitated solids are filteredoff and washed with hexane. The hexane is removed from the filtrate invacuo to give the crude methane sulfonate (1300 parts, wt) which can beused for the next stage.

An alternative purification method (resulting in a purer product and aless coloured final product of the later stages) is as follows: Thereaction mixture is diluted with t-butyl methyl ether (4000 parts, vol)and cooled to 5-10° C. With stirring and under nitrogen, water (2000parts, vol) is added and the aqueous layer adjusted to pH=1-2 withconcentrated hydrochloric acid. After 15 min., the layers are separatedand the aqueous layer extracted with t-butyl methyl ether (500 parts,vol). The combined organic layers are then washed with 1M hydrochloricacid (1000 parts, vol) and water (4×500 parts, vol.). The organic layeris dried (anhydrous sodium sulfate, 300 parts, wt), filtered andevaporated in vacuo to give purer methane sulfonate to use for the nextstage.

Example 1c Preparation of 2-Carboxy DGLA(2-Carboxy-icosa-8(Z),11(Z),14(Z)-trienoic acid

To absolute ethanol (10000 parts, vol) was added sodium methoxide 30%w/v in methanol (1370 parts, vol). At room temperature under nitrogen,diethyl malonate (1520 parts, wt) is added in a fast stream over 10-15min. and the mixture is stirred for a further 10-15 min. Crude GLAlcoholmethane sulfonate (1300 parts) is added in a fast stream over 10- 15 minand the mixture is stirred and heated under reflux for 3.5-4.0 hrs.under nitrogen. After cooling to room temperature, a solution made bydissolving potassium hydroxide 85% (1900 parts, wt,) in water (1000parts, vol) and then adding 95% ethanol (13000 parts, vol), is addedunder nitrogen. An exotherm occurs and the temperature of the reactionreaches 30-40° C. The mixture is stirred at room temperature for 4-5hrs. under nitrogen. The total reaction mixture is evaporated in therotary evaporator to remove the ethanol. The residue from theevaporation is dissolved in water (10000 parts, vol) and t-butyl methylether (10000 parts, vol) was added. The mixture is stirred and acidifiedunder nitrogen using 20% sulfuric acid (approx. 6000 parts, vol) (Maxtemp. 20° C.). After separating the layers, the organic layer is washedwith water (4×2000 parts, vol), dried (anhydrous sodium sulfate) andevaporated in vacuo to give an oil which crystallises on scratching togive crude 2-Carboxy DGLA (1170-1220 parts, wt, 88-92%).

Example 1d Preparation of DGLA (Icosa-8(Z),11(Z),14(Z)-trienoic acid

Crude 2-Carboxy DGLA (1200 parts, wt) is heated with stirring under avacuum of <30 mb at 140-160° C. Carbon dioxide is evolved and is removedby the vacuum. After 3-5 hrs, the emission of carbon dioxide ceases. Theflask is cooled to room temperature and nitrogen is let into thereaction vessel to give an oil (1000-1030 parts, wt, 95-98%).

Example 1e Purification of DGLA

The product of Example 1d (1000 parts, wt) is dissolved in HPLC (orequivalent) acetone (3550-3600 parts, vol) and with good stirring andunder nitrogen, a solution of lithium hydroxide hydrate (150 parts, wt.)in water (975 parts,vol) is added slowly over 30min. The mixture isstirred for a further 10min. Further acetone (3500-3600 parts, vol) isadded over 30 minutes with stirring and stirring is continued withcooling to 0 to -5° C. over a period of 2-3 hrs. The mixture is allowedto stir overnight at this temperature. The precipitated lithium salt isfiltered and washed with pre-cooled acetone and sucked dry. Theresulting solid is added in portions to a stirred cooled (0-10° C.)mixture of t-butyl methyl ether (6000 parts, vol) and 1M hydrochloricacid (6000 parts, vol) under nitrogen. The resulting organic layer isseparated, washed with water (4×500-750 parts, vol) and dried (Na₂SO4).After filtration, the solvent is evaporated and the resulting oil isheated under high vacuum (50-60° C., 0.1-1.0 mb) for several hrs toremove traces of solvent. There is obtained DGLA as a pale yellow oil(760-800 parts, wt, 76-80%). Decolourisation to a clear oil can beobtained by stirring the DGLA in 10 volumes of hexane in the presence ofchromatographic silica 35-70 μm particle size (20% by wt.) for 1 hr.,filtering and evaporating the solvent

Example 2 Preparation and purification of DPA(Docosa-7(Z),10(Z),13(Z),16(Z),19(Z)-pentaenoic acid

DPA was prepared in the same was as DGLA in Example 1 but from thestarting material EPA ethyl ester (ethyl ester of eicosapentaenoicacid).

In a first step, EPAlcohol (Icosa-5(Z),8(Z),11(Z),14(Z),17(Z)-pentaenol)was formed in a similar manner to Example la by replacing the GLA withEPA ethyl ester (1320 parts, wt).

In a second step, EPAlcohol methane sulfonate(Icosa-5(Z),8(Z),11(Z),14(Z),17(Z)-pentaenyl methane sulfonate) wasformed in a similar manner to Example 1b by replacing the GLAlcohol withsaid EPAlcohol (1091 parts, wt).

In a third step, 2-Carboxy DPA (2-Carboxy-docosa-7(Z),10(Z),13(Z),16(Z),19(Z)-pentaenoic acid) was formed in a similar manner to Example 1c byreplacing the GLAlcohol methane sulfonate with the said EPAlcoholmethane sulfonate (1391 parts, wt).

In a fourth step, DPA (Docosa-7(Z),10(Z),13(Z),16(Z),19(Z)-pentaenoicacid) was formed in a similar manner to Example ld by replacing the2-Carboxy DGLA with the said 2-Carboxy DPA with cooling of the acetonemixture to −15 to −20° C.

In a fifth step, the DPA is purified using the same lithium saltpurification method of

Example le but replacing the DGLA with said DPA.

1. A process for purifying a fatty acid,. which comprises contacting afatty acid with a lithium salt.
 2. A process according to claim 1wherein the lithium salt is lithium hydroxide.
 3. A process according toclaim 1 wherein the fatty acid is a C₁₄₋₂₄ fatty acid.
 4. A processaccording to any claim 1 wherein the fatty acid is selected from thegroup consisting of dihomo-gamma-linolenic acid (DGLA), eicosadienoicacid, docosapentaenoic acid (DPA), eicosatetraenoic acid (ETA) andeicosatrienoic acid.
 5. A process for purifying a fatty acid, whichprocess comprises: (a) reacting a fatty acid with a lithium salt in afirst solution and under conditions to allow formation of a precipitateof a lithium salt of the fatty acid; (b) isolating the precipitate; (c)dissolving the precipitate in a second solution which is capable ofgenerating two immiscible layers upon dissolution of the precipitate,the two immiscible layers being an organic layer and an aqueous acidiclayer; (d) separating the two immiscible layers formed upon dissolutionof the precipitate; and (e) evaporating the organic layer to isolate thepurified fatty acid.
 6. A process according to claim 5 wherein thelithium salt is lithium hydroxide.
 7. A process for preparing a fattyacid, which process comprises: (a) decarboxylating a malonic acidderivative of formula R—CH₂CH(CO₂H)₂, wherein R is a fatty acid residue,to form a fatty acid of formula RCH₂CH₂CO₂H; (b) subjecting the fattyacid thus prepared to a process for purifying a fatty acid as definedclaim
 5. 8. A process for extending the length of a fatty acid, whichprocess comprises: (a) reducing a fatty acid of formula R—CO₂H or afatty acid ester of formula R—CO₂R¹, wherein R is a fatty acid residueand R¹ is a C₁₋₆ alkyl group, to an alcohol of formula R—CH₂OH; (b)sulfonating the alcohol to form a sulfonate of formula R—CH₂OSO₂R²,wherein R² is a C₁₋₆ alkyl or C₆₋₁₀ aryl group; (c) reacting thesulfonate with a malonate ester derivative and hydrolysing the resultingproduct to form a malonic acid derivative of formula R—CH₂CH(CO₂H)₂; (d)decarboxylating the malonic acid derivative to form a fatty acid offormula R—CH₂CH₂CO₂H; and (e) subjecting the fatty acid thus prepared toa process for purifying a fatty acid as defined in claim
 5. 9. A processaccording to claim 8 wherein R is a fatty acid residue comprising from14 to 22 carbon atoms.
 10. A process according to claim 9 wherein Rcomprises from 18 to 20 carbon atoms.
 11. A process according to claim 8wherein R comprises 2 to 6 unconjugated olefinic groups.
 12. A processaccording to claim 8 wherein R is selected from the group consisting ofω-3 and ω-6 fatty acid residues.
 13. A process according to claim 8wherein R is selected from the group consisting of a C₁₇H₂₉ residue fromgamma-linolenic acid, a C₁₉H₂₉ residue from eicosapentaenoic acid, aC₁₇H₂₇ residue from stearidonic acid, a C₁₇H₃₁ residue from linoleicacid and a C₁₇H₂₉ residue from alpha-linolenic acid.
 14. A processaccording to claim 8 wherein the reduction in step (a) is carried outwith a solution of lithium aluminium hydride.
 15. A process according toclaim 8 wherein the malonic acid derivative of formula R—CH₂CH(CO₂H)₂ isused directly in the decarboxylation of step (d) without purificationand/or without crystallisation.
 16. A process according to claim 5wherein the fatty acid is a C₁₄₋₂₄ fatty acid.
 17. A process accordingto claim 5 wherein the fatty acid is selected from the group consistingof dihomo-gamma-linolenic acid (DGLA), eicosadienoic acid,docosapentaenoic acid (DPA), eicosatetraenoic acid (ETA) andeicosatrienoic acid.
 18. A process according to claim 7 wherein R is afatty acid residue comprising from 14 to 22 carbon atoms.
 19. A processaccording to claim 18 wherein R comprises from 18 to 20 carbon atoms.20. A process according to claim 7 wherein R comprises 2 to 6unconjugated olefinic groups.
 21. A process according to claim 7 whereinR is selected from the group consisting of ω-3 and ω-6 fatty acidresidues.
 22. A process according to claim 7 wherein R is chosen fromthe group consisting of a C₁₇H₂₉-residue from gamma-linolenic acid, aC₁₉H₂₉-residue from eicosapentaenoic acid, a C₁₇H₂₇-residue fromstearidonic acid, a C₁₇H₃₁-residue from linoleic acid and aC₁₇H₂₉-residue from alpha-linolenic acid.